CLDN4 and cholangiocarcinoma: Despite the efforts to identify CCA-specific markers in serum and bile, none of them proposed to date (mucins 1 and 5AC, metalloproteinases 7 and 9, claudin-4, IL6, IGF1, cytokeratin 19 fragments, etc.)has reached a level of specificity and sensitivity adequate for recommendation as useful tools in clinical practice [64, 65].